Showing papers in "Materials Science Forum in 2005"

The use of functionally graded poly-SiGe layers for MEMS applications

Abstract: It is difficult to meet all the different material and economical requirements posed to a MEMS structural layer that can be integrated with the electronics on the same substrate using a single layer process. Therefore a multilayer process, which uses a combination of a CVD crystallization layer and a high-growth rate PECVD bulk layer was developed. High-quality films with excellent electrical and mechanical properties can be obtained at low temperature (#450°C) and high deposition rates (~100 nm/min). Fine-tuning of the stress gradient is accomplished by the use of a top stress compensation layer, whose optimal thickness was estimated from an evaluation of the stress gradient profile over thickness. These layers have been used for processing a 10 µm thick poly-SiGe gyroscope on top of a standard 0.35 µm CMOS process.

Characterisation and Quantification of Complex Bainitic Microstructures in High and Ultra-High Strength Linepipe Steels

Abstract: This paper provides a detailed description of complex bainitic microstructures obtained during the recent development of low carbon linepipe steels with strengths in the range of X100 to X120 New experimental techniques based on a high resolution FEG-SEM and EBSD have been used to characterise and quantify the mixture of ultrafine bainitic ferrite and nanosize second phases in these steels It was found that the occurrence of incomplete transformation generates new, previously unexplored bainitic microstructures with a wealth of microstructural features that is beyond classification based on conventional concepts Clear differences in distributions of boundary misorientations and effective grain size were noted between upper, lower and granular bainites Based on these results a new classification scheme and definition of bainite is proposed

Mechanical Properties of Low-Temperature Bainite

Abstract: The mechanical properties of a bainitic microstructure with slender ferrite plates (20-65 nm in thickness) in a matrix of carbon-enriched retained austenite were characterized. The microstructure is generated by isothermal transformation at temperatures in the range 200-300°C. A yield strength as high as 1.5 GPa and an ultimate tensile strength between 1.77 to 2.2 GPa was achieved, depending on the transformation temperature. Furthermore, the high strength is frequently accompanied by ductility (£ 30%) and respectable levels of fracture toughness (

Effect of Austenite Grain Size on Martensitic Transformation of a Low Alloy Steel

Abstract: There are many empirical equations for predicting martensite start temperature (Ms) and the kinetics models of martensitic transformation of plain carbon and low alloy steels. The Ms temperature equations are only dependent upon the chemistry, while the martensite transformation kinetics models are based on the degree of undercooling below Ms temperature. However, the prior austenite grain size (AGS) is also expected to influence both Ms temperature and martensite transformation kinetics as it does in diffusive transformations. In this study, herefore, both Ms temperature and martensite transformation kinetics of a low alloy steel with different austenite grain sizes were investigated using a dilatometer. The new Ms equation and martensite transformation kinetics model including the AGS effect are proposed.

Texture Development in Pure Mg and Mg Alloy AZ31

Abstract: Texture evolution in pure Mg and Mg alloy AZ31 during deformation and annealing was investigated. The poor low temperature ductility can be attributed to both, insufficient shear systems and unfavorable deformation geometry. Static recrystallization was shown to proceed discontinuously despite little texture change. High temperature deformation was accompanied by dynamic recrystallization with similar texture development as during static recrystallization.

High Performance Bainitic Steels

Abstract: The choreography of atoms during the course of the bainite transformation has major consequences on the development of structure. In particular, the scale and extent of the structure is dependent directly on the fact that the atoms move in a disciplined fashion. This information can be exploited to develop unconventional alloys - for example, rail steels which do not rely on carbides for their properties, and the hardest ever bainite which can be manufactured in bulk form, without the need for rapid heat treatment or mechanical processing.

Transformation Textures in Steels

Abstract: The mechanisms by which textures can be inherited in transformed phases are discussed in the light of different transformation mechanisms. Possible origins of variant selection in the different cases are reviewed and classified. Evidence is presented for a hitherto unsuspected source of variant selection that arises from the stresses which are generated during transformation due to the presence of micro-segregation. Some model predictions show the potential effect of this phenomenon on textures in bainite or martensite.

Basalt Fiber Reinforced Hybrid Polymer Composites

Abstract: Short fiber (basalt, carbon, ceramic, and glass) reinforced polypropylene hybrid composites were investigated to determine their mechanical properties in case of different reinforcing fiber types. The composites were reinforced with fibers and were produced by hot pressing after hot mixing techniques. Composite properties such as flexural strength, stiffness, static and dynamic fracture toughness were measured. It was realized that the main damage modes of the composites are fiber pullout and debonding. It was also found that basalt fibers are the most sensitive to the lack of the treatment with additives. These results were supported by scanning electron micrographs taken of the fracture surfaces.

Atomic Mechanisms of Grain Boundary Motion

Abstract: We present results of atomistic computer simulations of spontaneous and stress-induced grain boundary (GB) migration in copper. Several symmetrical tilt GBs have been studied using the embedded-atom method and molecular dynamics. The GBs are observed to spontaneously migrate in a random manner. This spontaneous GB motion is always accompanied by relative translations of the grains parallel to the GB plane. Furthermore, external shear stresses applied parallel to the GB and normal to the tilt axis induce GB migration. Strong coupling is observed between the normal GB velocity vn and the grain translation rate v||. The mechanism of GB motion is established to be local lattice rotation within the GB core that does not involve any GB diffusion or sliding. The coupling constant between vn and v|| predicted within a simple geometric model accurately matches the molecular dynamics observations.

The Paths and Strategies for Increased Magnesium Applications in Vehicles

Abstract: The use of magnesium in motor vehicles today and in future depends on numerous technical and economical factors, though the cost factor is essential. How might use of magnesium in vehicles develop, what prerequisites are necessary, what R&D efforts are required ? These questions will be addressed as follows based on component and project examples. The main focus of the current magnesium applications can be seen in the drive train and in the interior. In the short term, these applications will be further expanded: doubling the amount is feasible. This will be supported by the currently developed Mg alloys with extended application potential (creep resistance). Mg components in the body, Mg-sheets and Mg-extrusions applications will appear initially in niche-market and premium vehicles. This can prepare the way for and eventually lead to greater use of Mg in volume-production models as part of a multi-material design concept. The essential prerequisites for such increased use of magnesium will be discussed.

Mechanical Properties of Forged Low Ni and C-Containing Co-Cr-Mo Biomedical Implant Alloy

Abstract: In order to examine the influence of variation in grain size and microstructure on mechanical properties of a low Ni and C-containing Co-29Cr-6Mo alloy for biomedical implant materials, the tensile properties and the dry friction wear characteristics of the wrought Co-29Cr-6Mo alloy without addition of Ni and C were investigated. The microstructure of as-forged alloys, consisting of fcc phase mixed with athermal hcp martensite, is found to become finer as the reduction during forging increases. The hot-forging processes for fabricating the fine microstructure with different grain sizes of 43, 11 and 3 μm has been successfully established under the condition that forging temperature of higher than 1273 K is strictly kept during forging. The tensile properties, such as the yield stress, the tensile stress and the elongation, are improved with decreasing the grain size and thereby increasing the volume fraction of the fcc phase in the microstructure. The dry wear resistance against the alumina ball is enhanced by decreasing the grain size.

Degradable, Multifunctional Polymeric Biomaterials with Shape-Memory

Abstract: An actual trend in polymer science is the design of materials which show multifunctionality meaning an unexpected combination of material functionalizations like the combination of biofunctionality, hydrolytic degradability, and shape-memory functionality. The development of multifunctional materials is often application-driven. I.e., certain demands of the modern society cannot be adequately solved with existing materials or classes of materials. These demands determine the necessary spectrum of functionalities to be provided by the respective materials . Besides finding high performance materials, material scientists concentrate on designing “intelligent” and “self-repairing materials”. In this context, materials showing a thermally induced shape-memory effect, such as metallic alloys or gels, have been studied intensively, and polymers showing shape-memory behavior have found growing interest. Shape-memory polymers are stimuliresponsive materials. Upon exposure to an external stimulus, they have the capability of changing their shape. A change in shape initiated by a change in temperature is called thermally induced shape-memory effect. The shape-memory effect results from the polymer’s structure in combination with a certain processing and programming technology. The polymer is processed into its permanent shape by conventional methods such as extrusion or injection molding. Afterwards, it is deformed and the desired temporary shape is fixed. The later process is called programming. Heating the programmed polymer above a temperature higher than the transition temperature Ttrans results in activating the shape-memory effect. As a consequence, the recovery of the memorized, permanent shape can be observed. The described effect is called a “one-way” shape-memory effect [1]. Materials which obtain their functionality after a functionalization process as described for shape-memory polymers are called functionalized materials. Taking into consideration the importance of polymeric materials in daily life, a very broad spectrum of possible applications for intelligent polymers opens up, covering an area from minimally invasive surgery to high performance textiles, and to self-repairing plastic components in all kinds of technical devices. Polyethylen covalently cross-linked by means of ionizing radiation has found broad application as heat shrinking film or tubing especially for the insulation of electric wires or as protection against corrosion of pipe lines [2-7]. These materials are marketed as “heat shrinkable materials”. Phase-segregated multiblock copolymers, mostly polyurethanes, can be found in literature being named with the generic term “shape-memory polymers” [8-17]. Thermoresponsive shape-memory properties is not confined to polymers. It has also been described for other materials, especially for ceramics and metallic alloys [18-27]. However, the mechanical properties of shape-memory alloys, like Nitinol  , can only be varied in a limited range. Their strongest restriction is the maximum deformation between permanent and temporary shape of only 8% [22]. Materials Science Forum Online: 2005-08-15 ISSN: 1662-9752, Vols. 492-493, pp 219-224 doi:10.4028/www.scientific.net/MSF.492-493.219 © 2005 Trans Tech Publications Ltd, Switzerland

Modeling Texture, Twinning and Hardening Evolution during Deformation of Hexagonal Materials

Abstract: Hexagonal materials deform plastically by activating diverse slip and twinning modes. The activation of such modes depends on their relative critical stresses, function of temperature and strain rate, and the orientation of the crystals with respect to the loading direction. For a constitutive description of these materials to be reliable, it has to account for texture evolution associated with twin reorientation, and for the effect of the twin barriers on dislocation propagation and on the stress-strain response. In this work we introduce a model for twinning which accounts explicitly for the composite character of the grain, formed by a matrix with embedded twin lamellae which evolve with deformation. Texture evolution takes place through reorientation due to slip and twinning. The role of the twins as barriers to dislocations is explicitly incorporated into the hardening description via a directional Hall-Petch mechanism. We apply this model to the interpretation of compression experiments both, monotonic and changing the loading direction, done in rolled Zr at 76K.

Increased Growth Rate in a SiC CVD Reactor Using HCl as a Growth Additive

Abstract: Hydrogen chloride (HCl) was added to a standard SiC epitaxial growth process as an additive gas. A low-pressure, hot-wall CVD reactor, using silane and propane precursors and a hydrogen carrier gas, was used for these experiments. It is proposed that the addition of HCl suppresses Si cluster formation in the gas phase, and possibly also preferentially etches material of low crystalline quality. The exact mechanism of the growth using an HCl additive is still under investigation, however, higher growth rates could be obtained and the surfaces were improved when HCl was added to the flow. The film morphology was studied using SEM and AFM and the quality with LTPL analysis, which are reported.

Residual Stress Mapping in Welds Using the Contour Method

Abstract: The contour method, a newly-invented sectioning technique for residual stress measurement, has the potential to measure the cross-sectional residual stress profile of a weld in a simple and time-efficient manner. In this paper we demonstrate the capability of the contour method to measure cross-sectional residual stress profiles, which are compared with neutron diffraction measurements and show excellent agreement. The results provide useful information for safetycritical design of welded components and optimization of welding parameters, and also illustrate the potential of the contour technique as a powerful tool for residual stress evaluation.

New Achievements on CVD Based Methods for SiC Epitaxial Growth

Abstract: The results of a new epitaxial process using an industrial 6x2” wafer reactor with the introduction of HCl during the growth have been reported. A complete reduction of silicon nucleation in the gas phase has been observed even for high silicon dilution parameters (Si/H2>0.05) and an increase of the growth rate until about 20 µm/h has been measured. No difference has been observed in terms of defects, doping uniformity (average maximum variation 8%) and thickness uniformity (average maximum variation 1.2 %) with respect to the standard process without HCl.

New Microalloyed Steel Applications for the Automotive Sector

Abstract: Developments related to the use of microalloy additions, primarily of Ti, Nb, and V, and controlled processing are reviewed to illustrate how steels with tailored microstructures and properties are produced from either bar or sheet steels for new automotive components. Microalloying additions are shown to control the necessary strengthening mechanisms to produce high strength materials with the desired toughness or formability for a specific application. Selected examples of direct cooled forging steels, microalloyed carburizing steels, and advanced high strength sheet (AHSS) steels are discussed.

Low Density of Interface States in n-Type 4H-SiC MOS Capacitors Achieved by Nitrogen Implantation

Abstract: A surface-near Gaussian nitrogen (N) profile is implanted into n-type 4H-SiC epilayers prior to a standard oxidation process. Depending on the depth of the oxidized layer and on the implanted N concentration, the density of interface states DIT determined in corresponding 4H-SiC MOS capacitors decreases to a minimum value of approx. 1010 cm-2eV-1 in the investigated energy range (EC-(0.1 eV to 0.6 eV)), while the flat-band voltage increases to negative values due to generated fixed positive charges. A thin surface-near layer, which is highly N-doped during the chemical vapour deposition growth, leads to a reduction of DIT only close to the conduction band edge.

Drift-Free, 50 A, 10 kV 4H-SiC PiN Diodes with Improved Device Yields

Abstract: The path to commericializing a 4H-SiC power PiN diode has faced many difficult challenges. In this work, we report a 50 A, 10 kV 4H-SiC PiN diode technology where good crystalline quality and high carrier lifetime of the material has enabled a high yielding process with VF as low as 3.9 V @ 100 A/cm2. Furthermore, incorporation of two independent basal plane dislocation reduction processes (LBPD 1 and LBPD 2) have produced a large number of devices that exhibit a high degree of forward voltage stability with encouraging reverse blocking capability. This results in a total yield (forward, 10 kV blocking, and drift) of >20% for 8.7 mm x 8.7 mm power PiN diode chips—the largest SiC chip reported to date.

Microalloying of Cold-Formable Multi Phase Steel Grades

Abstract: Microalloying elements like Al, B, Nb, Ti ,V can be used to optimise the microstructure evolution and the mechanical properties of advanced high strength steels (AHSS) Microalloying elements are characterised by small additions

Optical Characterization of Deep Level Defects in SiC

Abstract: Deep levels in 4H- and 6H-SiC are characterized by FTIR spectroscopy. Vanadium, chromium and the silicon vacancy related center are listed together with the unidentified defects with emission and absorption in the near IR region. We suggest the UD-1, UD-3 and I-1 to be impurity related while the UD-2 and UD-4 to be intrinsic defects based on annealing behavior and the possibility to create the defect with irradiation. We have also tentatively assigned a new defect center around 1.0 eV to the carbon vacancy-antisite pair instead of the earlier assignment to the UD- 2 defect in 4H-SiC. We have shown that to get more information about the SiC samples a combination of absorption and luminescence techniques are very useful. Further, the use of below bandgap selective excitation is necessary to obtain more information about the defects present in the sample. FTIR absorption and luminescence measurements are useful tools to characterize deep levels important for both semi-insulating material as well as low doped conducting material where the free carrier lifetime is limited by deep levels.

Structure and Morphology of 4H-SiC Wafer Surfaces after H2-Etching

Abstract: Commercial on-axis wafers of 4H-SiC(0001) were etched in a standard reactor for chemical vapor deposition (CVD) using molecular hydrogen flux in order to improve the structure and morphology of the surface. The substrate temperature during etching was varied from 1400 to 1600°C. Characterization of the surface morphology was performed using optical and atomic force microscopy (AFM). Low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) were also used to examine the surface structure and chemical composition of the samples. The sample of best quality was obtained for an etching temperature of 1400°C. Its surface is ° × 30 ) 3 3 ( R reconstructed and covered by an ordered “silicate” layer. Increasing the substrate temperature during etching to 1500°C leads to enhanced step-bunching and the formation of macroterraces. At 1600°C distinct depressions appear on the surface, presumably from etching of structural defects such as screw dislocations. Subsequent annealing at 1000°C in ultra-high vacuum (UHV) removes the surface oxide and produces the ° × 30 ) 3 3 ( R surface phase of clean 4HSiC( 0001).

Orientation Maps Derived from TEM Diffraction Patterns Collected with an External CCD Camera

Abstract: An automatic crystallographic orientation indexing procedure is developed for transmission electron microscopes. The numerical identification is performed by mapping the spot diffraction patterns with pre-calculated templates. The diffraction patterns are acquired thanks to an external CCD camera that points to the fluorescent screen through the TEM window. Orientation maps with spatial resolution better than 10 nm were obtained with this low cost equipment.

Investigations in the Magnesium-Tin System

Abstract: Currently most magnesium alloys are based on the Mg-Al system and this system is reasonably well developed. Nevertheless, the alloy system has some disadvantages – while having excellent castability and adequate room temperature mechanical properties Mg-Al alloys show poor creep resistance. This has led to investigations in other magnesium based systems; magnesium forms alloys with a large number of elements and, indeed a number of magnesium systems show good creep resistance. In this work, the Mg-Sn system has been chosen for study. The Mg-Sn system shows some positive characteristics in terms of potential creep resistance; a high temperature stable phase is formed with magnesium Mg2Sn (melting point: 770°C) and this phase forms as a line compound (71wt% Sn), which is typically distributed around the grain boundaries. The effect of the addition of a range of ternary elements on the microstructure and creep properties has been studied.

SiC Materials and Technologies for Sensors Development

Abstract: Silicon Carbide has proven its strong interest for power and high frequency devices but it also has superior characteristics for application in the sensors and MEMS fields. The characteristic requirements of the starting material are different from that of power devices since the level of defects is not so critical while the layer stress is important especially in 3C-SiC on Si. The keyprocess for MEMS fabrication is the etching, which is progressing thanks to ICP process improvements. A perfect control of the etching step could allow the obtention of nano-resonators in SiC with fairly superior characteristics to the Si ones. Other electrical sensors for high temperature application such as gas sensors or Hall sensors have been also successfully developed taking profit of the deep etching process improvement and high temperature contact developments.

An Analysis of the Development and Applications of Current and New Mg-Al Based Elevated Temperature Magnesium Alloys

Abstract: The current status of research and application in the AZ(Mg-Al-Zn), AS(Mg-Al-Si), AE(Mg-Al-RE), AX(Mg-Al-Ca), ACM or MRI（Mg-Al-Ca-RE）and AJ（Mg-Al-Sr）series elevated temperature magnesium alloys are reviewed, will special attention paid to the effects of alloying elements and the control of second phases. The existing problems on the development of elevated temperature magnesium alloys are discussed.

High Quality Ceramic Coatings Sprayed by High Efficiency Hypersonic Plasma Spraying Gun

Abstract: This paper introduced the structure of the high efficiency hypersonic plasma spraying gun and the effects of hypersonic plasma jet on the sprayed particles. The optimised spraying process parameters for several ceramic powders such as Al2O3, Cr2O3, ZrO2, Cr3C2 and Co-WC were listed. The properties and microstructure of the sprayed ceramic coatings were investigated. Nano Al2O3-TiO2 ceramic coating sprayed by using the high efficiency hypersonic plasma spraying was also studied. Compared with the conventional air plasma spraying, high efficiency hypersonic plasma spraying improves greatly the ceramic coatings quality but at low cost.

Application of Niobium in Quenched and Tempered High-Strength Steels

Abstract: This article offers an overview on the ways and means of the application of Nb in quenched and tempered high-strength steel plates. Thereby, the outstanding role of Nb to control the austenite microstructure during rolling or heat treatment and to contribute to effective precipitation hardening is discussed. It is shown, that Nb is very effective to retard the transformation processes during quenching. For high-strength constructional steels with up to 1100 MPa yield strength and for wear resistant steels, the improvement in strength and toughness properties in Nb microalloyed steels as compared to Nb-free steels is pointed out. A remarkable effect is the improvement in toughness and brittle fracture resistance due to a very fine microstructure and finely dispersed Nb carbonitrides in a martensitic microstructure. Fields of application for the new Nb microalloyed high-strength structural steels are presented too.

Analysis of Intragranular Misorientation Related to Deformation in an Al-Mg-Mn Alloy

Abstract: Intragranular misorientation reflects strain generated during deformation with dislocation glide. The SEM/EBSP (scanning electron microscope/ electron back scatter diffraction pattern) technique provides is “kernel average misorientation (KAM)” as the most appropriate quantity to evaluate the strain or the stored energy for a given point. The KAM is defined for a given point as the average misorientation of that point with all of its neighbors. In the present paper two analyses of the intragranular misorientation using the SEM/EBSP technique for a cyclic deformation at room temperature and a high temperature deformation in an Al-Mg-Mn alloy are reviewed.